Dental porcelain, a material composed of feldspar, quartz and kaolin, is commonly used in fabricating dental restorations. Full porcelain crowns, or restorations, although aesthetically highly desirable have generally been employed in the past only for coverage of anterior teeth, where aesthetics is the prime consideration. The reason for this is because porcelain is inherently a structurally weak material. One technique for making a porcelain crown involves the use of a malleable foil which is ultimately removed from within the fused porcelain.
Because of this deficiency with full porcelain restorations, a porcelain veneer cast metal crown has been extensively employed. This crown has a relatively thick noble metal substructure formed from casting an investment of a wax or plastic pattern of the prepared tooth. Dental porcelain is then applied in layers over part, or all, of the substructure and fired at high temperatures to form the veneer. Since the thickness of the substructure is substantial, material costs in such a cast metal crown are high due to the use of large amounts of expensive noble metals. These type crowns are not totally acceptable either because the thickness of the metal substructure minimizes the permissible thickness which may be employed for the porcelain veneer. Of course, any exposure of the metal substructure will detract from the aesthetics of the restoration. This can result, for example, if the porcelain veneer were to be chipped or if the porcelain veneer were too thin as might be the case if the cast metal substructure is too thick. Finally it will be appreciated that the manufacture of such a restoration, with the waxing, casting and machining that is required, is very expensive because of its being labor intensive.
In an attempt to address some of the above deficiencies, so-called "foil" systems have been developed to produce a reinforced porcelain jacket crown restoration. Unfortunately, the crowns produced by such techniques are also structurally weak and fragile Furthermore, during an intermediate phase of this production technique a weak and malleable structure is produced which is easily crushed, even accidentally. This results in substantial amounts of wasted time and materials which consequently has an adverse impact on production economics.
Briefly, the "foil" technology involves the use of a deformable or malleable noble metal foil which is conformed to the shape of the die of the prepared tooth. A bonding agent may be employed on the outer surface of the conformed foil and then porcelain is applied over such an understructure. The conforming foil is very easily deformed and great care must be exercised in its utilization, especially during the application of the porcelain. Furthermore, because of this deformability and inherent weakness in the substructure, the final crown likewise is subject to deficiencies because of its poor strength. Additionally such "foil" technology is not adaptable to forming dental bridge restorations.
U.S. Pat. Nos. 4,459,112 and 4,273,580 exemplify "foil" systems. It will be readily apparent from the teachings of these patents that the copings which are formed have virtually no crush-resistance whatsoever. The technique disclosed in U.S. Pat. No. 4,459,112 is likewise labor intensive because of the care which must be exercised in forming the pleated, or umbrella-like, structure and also, in following the preferred embodiment, in forming small slotted openings through portions of the foil.
Thus from the foregoing, it will be seen that there is a need in the art to provide for an improved crown which can be manufactured by an economical process. In order to be economical there is a need that this process eliminate previously required steps including eliminating the need for waxing, casting and machining of a substructure. There is also a need in the art that the method not require the use of thick substructures to thereby increase the economics of the system by greatly minimizing the amount of noble metal which need be employed. Additionally, in contrast to the "foil" systems which produce an easily crushed coping, or substructure, there is a need in the art to provide for a process wherein the coping is crush-resistant. In that way the coping can be more easily handled during crown manufacture and, ultimately results in the formation of a stronger, more durable final dental restoration. Such strength would also allow the process to be adapted to forming a bridge. Additionally, there is a need in the art to provide for a process which does not require the additional step of slitting portions of a "foil".